As a method for measuring a concentration of nitrogen impurities in a mixed gas containing argon and oxygen as main components, a method in which the mixed gas is introduced into an electric discharge tube, wherein the pressure is no more than the atmospheric pressure, to measure the emission intensity of light emitted by electric discharge, the light being particular to nitrogen, and the oxygen concentration of the mixed gas is measured, and then the nitrogen concentration obtained from the emission intensity of the light is corrected based on the oxygen concentration of the mixed gas, thereby accurately measuring the concentration of nitrogen impurities contained in the mixed gas, is suggested (For example, Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2005-249551